1. Field of the Invention
The subject invention relates to storage tanks for liquids, such as liquid hydrocarbon products, and, more particularly, to floating roof structures for liquid storage tanks.
2. Description of the Prior Art
Floating roofs for liquid storage tanks are well known and are in widespread use all over the civilized world. The most typical application of these floating roofs is in the storage of liquid hydrocarbons, but no limitation to that application is intended herein.
Early prior-art proposals utilized a single floating deck, as may for instance be seen from U.S. Pat. No. 1,493,091, by J. H. Wiggins, issued May 6, 1924. These early constructions were subject to capsizing, which tendency was attempted to be alleviated by providing a large number of drainpipes designed to act as trusses for maintaining the deck in a horizontal position. This solution was not satisfactory from a design and safety standpoint. The single-deck construction was, however, retained for smaller installations, as may be seen from German Patent 607,596, by Servan Georges Cantacuzene, dated Jan. 30, 1932. Despite a provision of annular floats, no reliable safeguards were present against capsizing of the floating deck in the case of heavy winds or rain water loads.
The floating roof was thus made of stiffer construction and provided with a plurality of buoys located below the deck and submerged in the covered liquid, as may be seen in U.S. Pat. No. 2,036,372, by J. A. Stough, issued Apr. 7, 1936. In practice, the presence of these submerged buoys encouraged the formation of gas pockets which formed under the floating roof. In consequence, the deck corroded more rapidly, which led to explosions in some instances and in others caused capsizing of the roof when loaded with water, and the like accidents.
Accordingly, a construction was developed in which the buoys were located on top of the floating deck, to prevent sinking of the deck in the case of leaks in the deck structure. As may be seen from U.S. Pat. No. 2,359,416, by H. Hammeren, issued Oct. 3, 1944, these above-deck buoys typically had the form of rectangular parallelepipeds (prisms with six rectangular faces). This above-deck buoy configuration was unfavorable in practice in that it led to undesired movements and capsizing of the floating roof during heavy storms and impeded a rapid removal of heavy atmospheric precipitation.
As may be seen from U.S. Pat. No. 2,586,856, by C. M. Orr et al, issued Feb. 26, 1952, and U.S. Pat. No. 3,269,583, by A. F. Fino, issued Aug. 30, 1966, double deck constructions became favored for larger floating roof designs. This construction had the advantage that a smooth top surface could be provided on the roof, despite a high floating capability. In practice, this advantage was, however, typically outweighed by the more expensive and heavier construction. Accordingly, attempts were made to construct single-deck floating roofs with only peripheral pontoon structures, as seen in U.S. Pat. No. 2,843,289, by R. C. Ulm, issued July 15, 1958, and U.S. Pat. No. 2,913,138, by J. M. Swick, issued Nov. 17, 1959. However, a limitation to peripheral pontoons typically was only suitable for smaller designs.
Accordingly, proposals with below-deck buoys again came into vogue, as may be seen from U.S. Pat. Nos. 3,288,322 and 3,362,562, by D. D. Marshall et al, issued Nov. 29, 1966, and Jan. 9, 1968, respectively.
More current prior-art proposals revert to the above-deck buoy construction, employing a large number of buoys distributed over a single deck on top thereof. These proposals advocate the use of buoys of either cylindrical or rectangular prismatic configuration. In the cylindrical configuration, the circular surface traced by the generatrix is in a plane parallel to the single deck, while the elemental lines of the cylindrical configuration are perpendicular to the deck. In the prismatic configuration, the buoy has four rectangular sides perpendicular to the deck and a rectangular top side. In both types of configurations, the buoys have flat top sides extending in parallel to the floating deck. While the cylindrical configuration would be aerodynamically and hydrodynamically more favorable than the prismatic configuration, no preference is in the prior art given to the former over the latter. This is presumably due to the fact that a cylindrical configuration, in the orientation under consideration, presents an improvement in terms of air resistance by only a factor of about two over rectangular prismatic configurations. Accordingly, a cylindrical configuration of the above mentioned type for the above-deck buoys does not reliably solve the problems raised by rectangular prismatic buoy configurations.
However, there persisted a prejudice in the art to use prismatic buoys with rectangular sides that extended perpendicularly to the deck, or cylindrical buoys whose elemental lines extended also perpendicularly to the deck. With such configurations, the incremental liquid displacement and thus the incremental buoyancy remains constant with progressing submersion of the buoy, since the cross section of the buoy in parallel to the deck is constant from the bottom to the top of the buoy. Requisite buoy volumes and available buoyancies became thus easier to calculate. However, this convenience was bought at an exorbitant price, as buoys of this configuration rendered the floating roof vulnerable to the impact of high winds and wind-driven rain and sleet. In particular, high wind action on these prior-art buoys promoted undulating motion of the deck structure with consequent fatigue cracking of the deck seams or welds.
In a similar vein, the need arose in practice to provide for a support of the floating roof structure not only during construction, but also upon advanced diminution of the liquid level in the tank, so as to avoid a settling of the roof on the bottom of the tank. As may be seen in the above mentioned U.S. Pat. No. 2,586,856, by C. M. Orr et al, a typical prior-art support design included adjustable supporting posts releasably retained in tubular members which were attached to both decks of a double-deck floating roof structure. In the case of single-deck floating roofs, the stability for the roof supports provided in the latter case was not available from the roof structure itself. Accordingly, as may be seen from U.S. Pat. No. 2,464,803, by F. L. Goldsby et al., issued Mar. 22, 1949, special bracing plates and bracing rod arrays were required for lending some stability to the roof support pipes. This rendered the floating roof design more cumbersome and expensive and impeded construction on a prefabricated basis.
To provide a floating cover that could accommodate wave movements in the liquid, some prior-art proposals, such as British Patent Specification No. 876,436 by Shell, advocate the use of a thermoplastic foil for the cover. This foil is maintained floating by concentric or parallel tubular pontoons. Covers of this type are difficult to install and difficult to maintain in a gas and liquid tight condition. Also, due to the flexible nature of thermoplastic foil materials, a partial immersion of the tubular pontoons or of their silhouettes with attendant gas pocket formation cannot generally be avoided.
For these and other known reasons, prior-art floating roofs have difficulties meeting prevailing API requirements relating to punctured decks and rainfall load carrying capacity.